The present invention relates to methods for increasing efficiency and throughput in cross-flow devices for heating and cooling solids, and, more particularly, to methods for reducing solids residence time and reducing gas leakage between adjacent heating and/or cooling zones in such devices.
Cross-flow devices for heating and cooling solids typically use a single forced draft or induced draft blower, or a combination of two or more blowers using forced draft and induced draft together, to produce a flow of heating or cooling gas through a gas permeable bed of solids. In a typical device such as, for example, a traveling grate of the type used in iron ore pelletizing, incoming cool solids are heated by a cross-flow of hot gas and then cooled by a cross flow of incoming cool gas to recover the sensible heating of the solids. For convenience, the terms "preheat zone" and "heating zone" will be used to refer to the portions of the device in which the solids are heated by cross flows of relatively hotter gas, and the term "cooling zone" will refer to the portions of the device in which the solids are similarly cooled. The off-gas from the cooling zone may be further heated to bring it to a desired temperature and then passed through the incoming bed of solids in the preheat or heating zone. In some applications, the off gas from the heating zone may be passed through the incoming solids bed prior to the heating zone to preheat the solids and to recover the available sensible heat from the gas.
To effect the flow of the gases from zone to zone, a variety of flow arrangements can and have been used. The blowers have heretofore usually been positioned away from hot gas areas, typically at the cooling zone gas inlet and/or the heating zone gas outlet. This method of moving the gases from zone to zone without intermediate blowers necessarily results in different pressure profiles within the zones, due to pressure drops in the ductwork as well as through the solids bed. The differential pressure between corresponding bed locations in the various zones causes air flow and leakage of gas between zones through the solids, along the path of the bed. This interzone gas leakage results in both energy lossess and product losses, reducing thermal efficiency of the apparatus and decreasing product yield.
The gas leakage between zones can be reduced by providing a space between zones along the path of travel of the solids. This crude seal works on the principle that for a given differential pressure the gas flow rate, in this case the interzone leakage, decreases as the length of the flow path increases. Thus, by increasing the distance between zones and sealing the void space above and below the bed, the leakage can be reduced. This type of seal, however, reduces the throughout capacity of the device because active traveling grate area must be used to provide the seal space between zones.
The heating or cooling of a bed of solids in a cross-flow device involves contact between the solids and the cross-flowing fluid for some finite period of time. "Residence time" is the time required for all the solids at a given point along the bed, (usually the discharge point or the point at which the bed leaves a heating or cooling chamber or zone of the device) to come to the desired temperature. The residence time necessary for any particular process depends, of course, on numerous heating and cooling variables. It will be appreciated that the solids closest to the gas discharge side of the bed will require the longest time to heat or cool. This is because the solids on the gas inlet side of the bed are the first to be exposed to the incoming gas, and the initial heat transfer between the incoming gas and these solids reduces the driving force for heat transfer to or from the solids as the gas flows to the gas outlet side of the bed. For example, the normal residence time in a heating zone is the sum of the time required for the incoming hot gas to reach the outlet side of the bed at the desired temperature plus the time required for heat transfer between that gas and the outlet side solids. As a consequence, the heating or cooling chamber or zone occupies a portion of the device where the solids near the gas inlet side of the bed are at the desired condition while those at the gas outlet side are still being heated or cooled.